The present invention relates to an electric double layer capacitor which utilizes the principles of an electric double layer in the interface between a polarized electrode and an electrolyte, and more particularly to an electric double layer capacitor having improved polarized electrodes.
There have heretofore been well known electric double layer capacitors which include electrodes made of fine powder of carbon. The conventional electric double layer capacitors comprise two electrodes divided by an ion-permeable membrane and each made of a kneaded paste of fine carbon fiber dispersed in an electrolytic solution, and current collectors electronically connected to the respective electrodes. The electric double layer capacitors can be charged or discharged when a voltage is applied or removed from between the current collectors.
Various attempts have been made to increase the capacitance of such electric double layer capacitors. For example, Japanese Laid-Open Patent Publication No. 1(1989)-102914 discloses an electric double layer capacitor which has electrodes each made of a mixture of fine powder of active carbon of phenolic resin and an electrolytic solution of sulfuric acid. The fine active carbon powder has an average particle diameter ranging from 1.5 .mu.m to 4 .mu.m and a specific surface area ranging from 1,000 to 1,500 m.sup.2 /gram The active carbon powder of the above nature is highly effective to increase the electrostatic capacitance of the electric double layer capacitor, and reduce the equivalent series resistance thereof.
The process of manufacturing fine active carbon powder of phenolic resin is however complex, and requires a pulverizer for pulverizing the material into active carbon particles whose diameter ranges from 1.5 .mu.m to 4 .mu.m. Accordingly, the manufactured fine active carbon powder is expensive.
Since the particle diameters of the fine active carbon powder are very small, the fine active carbon powder cannot easily be handled as it tends to fly up with weak air streams when the electrodes of the electric double layer capacitor are fabricated. In addition, the maximum specific surface area of the fine active carbon powder of phenolic resin is 2,000 m.sup.2 /gram, imposing a certain limitation on the electrostatic capacitance that can be achieved by the electric double layer capacitor.
Another electric double layer capacitor disclosed in Japanese Laid-Open Patent publication No. 63(1988)-78514 comprises electrodes of fine active carbon powder of coke.
The fine active carbon powder of coke is manufactured by pulverizing coke into particles of 100 mesh or less in streams of nitrogen Therefore, the manufacturing process is complex and time-consuming. Because the particles of the produced fine active carbon powder are not spherical in shape, the electrostatic capacitance of the electric double layer capacitor cannot be increased as much as expected even if the fine active carbon powder has a relatively high specific surface area.